Never played FPL before but I’m assuming all out attack to do good. centre backs and defensive midfielders pointless as most of them don’t regularly score/assist.

It’s useful to have a a couple of defenders from Chelsea or spurs as they often keep clean sheets, so you get six points.
Even better if you have defenders that also get goals and assists like Cahill, Alonso, Coleman, Baines etc, especially if they take penalties, free kicks and corners.
But obviously players like Sanchez, Hazard, De Bruyne and Lukaku will often get you points as well but that’s why they are so expensive.